An electric double layer capacitor (hereinafter referred to as “EDLC”) is a condenser utilizing physical accumulation of charge based on an electric double layer, and has a number of advantages over chemical-reaction-based secondary batteries, such as an ability of high-output quick charge/discharge, and non-polluting properties. It is expected that electric double layer capacitors will be applicable to various uses, including use of compact electric double layer capacitors for electronic devices, and use of large electric double layer capacitors in automobile batteries. An electrode film for an EDLC is usually comprises a powder electrode such as an activated carbon; an electrically conductive material; a binder such as a fluorocarbon resin; and the like (e.g., JP 2004-2105 A).
The performance of an EDLC is influenced by the amount of an electrolyte impregnated in this electrode film. JP 4-47449 B describes destroying of the surface of a sheet-like electrode, especially a dense and smooth layer containing a large amount of a tetrafluoroethylene resin (polytetrafluoroethylene; hereinafter referred to as “PTFE”), to provide the surface of the sheet-like electrode with flaws in order to improve the electrolyte impregnation capacity of the sheet-like electrode.
However, use of this technique may cause the sheet-like electrode to be destroyed, leading to powder falling from the sheet-like electrode and shape changes (thickness, density, and the like), resulting in deterioration of the electric stability of the sheet-like electrode or an EDLC using the sheet-like electrode.
On the other hand, there is a known technique in which an electrode film for an EDLC is irradiated with an electromagnetic wave to improve its properties. For example, JP 2008-205275 A describes that, by irradiating an activated carbon as a material for an EDLC electrode with an electromagnetic wave in an atmosphere of hydrogen or the like, an oxygen-containing functional group present on the surface of the activated carbon is released, and that the thus produced material for an EDLC electrode can be used for production of a high-capacity EDLC having stable performance.
JP 2002-237434 A describes a technique in which, in order to solve problems caused by attachment, in a layer-forming manner, of a binder such as PVDF on the surface of an activated carbon during production of an electrode body, an electrode functional material prepared by kneading a powder electrode material, a binder, and a solvent is applied to a surface of a metal foil, and is dried, and then dried material is irradiated with laser to remove the layer of the binder attached, in a layer-forming manner, to the surface of the active substance, while maintaining the binder function of each particle. The document mentions an excimer laser as an example of this laser.
However, the techniques disclosed in JP 2008-205275 A and JP 2002-237434 A are not intended for improvement of the electrolyte impregnation capacity of an EDLC electrode film.